According to a newspaper article, last year I believe, the Army lost a dummy tank in Wales. Some OTC people discovered it in a gorge after a severe storm. Don't quote me but the cost was something in the region of £5000.
I had a look at an RAF owned dummy tank on exercise and I am sure myself and a few friends could knock up a better dummy tank, on a Friday afternoon before happy hour, for as little as £500.
I can assure you that the money would be spent behind the bar on alcohol and a jumbo sized packet of Ready Salted crisps.
(I am also contemplating making a cannon that fires crap beer cans). Not sure how much this will cost.